Robot and method for bending orthodontic archwires and other medical devices

ABSTRACT

A robotic bending apparatus for bending archwires and other types of elongate, bendable medical devices into a desired configuration includes a first gripping tool and a moveable gripping tool. The first gripping tool can be either fixed with respect to a base or table for the robot or positioned at the end of robot am. The moveable gripping tool is mounted to the end of a moveable robot arm having a proximal portion also mounted to the base. The robot preferably comprises a six axis bending robot, in which the distal end of the moveable arm can move relative to the fixed gripping tool about three translational axes and three rotational axes. The gripping tools preferably incorporate force sensors which are used to determine overbends needed to get the desired final shape of the archwire. The robot may also include a resistive heating system in which current flows through the wire while the wire is held in a bent condition to heat the wire and thereby retain the bent shape of the wire. A magazine for holding a plurality of straight archwires needing to be bent and a conveyor system for receiving the wires after the bending process is complete are also described. The robot bending system is able to form archwires with any required second and third order bends quickly and with high precision. As such, it is highly suitable for use in a precision appliance-manufacturing center manufacturing a large number of archwires (or other medical devices or appliances) for a distributed base of clinics.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of Ser. No. 09/834,967filed Apr. 13, 2001, now U.S. Pat. No. 6,612,143 the contents of whichare incorporated by reference herein.

BACKGROUND OF THE INVENTION

A. Field of the Invention

This invention relates to a robot and method for automatically bendingorthodontic archwires, retainers, or other orthodontic or medicaldevices into a particular shape.

B. Description of Related Art

In orthodontics, a patient suffering from a malocclusion is treated byaffixing brackets to the surface of the teeth and installing an archwirein the slots of the brackets. The archwire and brackets are designed togenerated a customized force system that applies forces to teeth, bywhich individual teeth are moved relative to surrounding anatomicalstructures into a desired occlusion. There are two approaches todesigning an appropriate force system for a patient. One is based on aflat archwire and customized brackets, e.g., Andreiko et al., U.S. Pat.No. 5,447,432. The other is based on off-the shelf brackets anddesigning a customized archwire that has complex bends designed to moveor rotate the teeth in the desired direction. Traditionally, the latterapproach has required manual bending of the archwire by theorthodontist.

Machines for bending orthodontic archwires have been proposed in theprior art. Andreiko et al. describes an apparatus that takes a straightarchwire and imparts a simple planar arcuate curvature to the wire. Thewire is customized in the sense that the shape of the arc is designedfor a particular patient, but the wire bending apparatus described inAndreiko et al. is limited to a customized bracket approach toorthodontics. In particular, the Andreiko et al. wire bending apparatuscannot produce any complex and twists bends in the wire, e.g., bendsrequiring a combination of translation and rotational motion.

The patent to Orthuber et al., U.S. Pat. No. 4,656,860 describes abending robot for bending archwires. A robot as described in the '860patent was developed by the predecessor of the assignee of the presentinvention and used experimentally for several years, but never widelycommercialized. The robot consisted of two characteristic designfeatures: a bending cone that could move forwards and backwards to bendthe wire, and a rotating cone that could twist the wire. As such, itcould only apply torque or bends over the two main axes of a crosssection of a rectangular shaped wire. Since the portion of the wireextending beyond the cone is free and unconstrained, the robot had nocontrol as to the effective deformation of the wire. Additionally, aseries of three twists and two bends were typically required by a robotin accordance with the '860 patent to shape an archwire so that it wouldfit in the slots of two adjacent brackets. This series of twists andbends required as much as 5 mm of wire length between adjacent brackets.This length of wire is greater than that available for closely spacedteeth, such as the lower front teeth. To avoid this situation, the robotbent a twisted portion of the wire, which provoked uncontrolledrotational motion in the wire.

The design of the '860 patent also has other shortcomings: it providesno means for measuring forces imparted by the wire since one end of thewire is free and the wire is gripped immediately below the bendingpoint. The robot had no effective feedback mechanism for detecting howthe wire in fact was bent after a particular bending or twistingoperation was performed. As the free end of the wire is not constrainedor held in any manner, there is no ready way to heat the wire as it isbeing bent in order to fix the shape of the bend in a wire made from ashape memory material. Consequently, shape memory alloy wires made withthe '860 patent were subject to a separate heating treatment in aseparate thermal device.

The present invention presents a substantial improvement to the robot ofthe '860 patent. The invention also provides for much greaterflexibility in the design and manufacture of archwires than thatdisclosed by the Andreiko et al. patent. In particular, the presentinvention enables the manufacture of custom, highly accurate orthodonticarchwires. Such wires are ideally suited to an archwire-basedorthodontic treatment regime based on standard, off-the-shelf brackets.The invention is also readily adaptable to bending other medicaldevices, including implants such bone fixation plates, prostheses,orthotic devices, and even surgical tools.

SUMMARY OF THE INVENTION

In a first aspect, a bending apparatus or machine is provided forbending an orthodontic appliance, such as a retainer or archwire, into adesired configuration. While the orthodontic device is described asbeing an archwire in the illustrated embodiment, other types of medicaldevices are contemplated as the type of article capable of being bent bythe robot. Examples of such medical devices are prostheses, orthoticdevices, implants, fixation plates, spectacle frames, and surgicaldevices such as a reamer for root canals.

The bending apparatus or machine may take the form of a robot mounted toa base or table support surface. A first gripper tool is provided. Thistool can either be fixed with respect to the base or may be incorporatedinto a moveable arm. The first gripping tool has a first grippingstructure for holding the archwire or other medical device. The bendingapparatus includes a moveable arm having a proximal portion mounted tothe base a distance away from the first gripper tool and a free distalend portion. The moveable arm is constructed such that the free distalportion of the moveable arm is capable of independent movement relativeto the first gripper tool along at least one translation axis and aboutat least one rotation axis. In an illustrated embodiment, the moveablearm has a set of joints which allows the distal end of the arm to movein 6 degrees of freedom −3 orthogonal translational axes and 3orthogonal rotational axes. However, depending on the nature of themedical device and the required bends to form in the device, a lessernumber of degrees of freedom may be appropriate, reducing the cost andcomplexity of the bending apparatus.

A second gripping tool is mounted to the distal portion of the moveablearm. The second gripping tool has a gripping structure for gripping thearchwire. Thus, the archwire is gripped by the first and second grippingtools, with the second, moveable gripping tool capable of motionrelative to the first gripping tool along at least one translationalaxis and at least one rotational axis.

The robot further includes a control system operative of the moveablearm and the first and second gripping tools so as to cause the first andsecond gripping tools to grip the archwire while the gripping tools areseparated from each other and to cause the second gripping tool to moveabout at least one of the rotational axis and translation axis tothereby bend the archwire a desired amount. Preferably, the controlsystem reads an input file containing information as to the shape of thearchwire (or location of bending points along the wire) and responsivelyoperates the moveable arm and first and second gripping tools to form aseries of bends and/or twists in the archwire.

The nature of the bends in the archwire will be dictated by theorthodontic prescription and the type of force system that theorthodontist has chosen for the patient. Complex bends involving acombination of bends and twists are possible with the robot. For suchcomplex bends, it has been found that a six-axis robot, in which thesecond gripping tool is capable of movement relative to the firstgripping tool about three translation axes and three rotation axes, is apreferred embodiment.

Orthodontic archwires and other medical devices may have elasticproperties such that when a certain amount of force is applied to theworkpiece, it returns to its original configuration at least to somedegree. What this means is that when a certain bend is formed in thewire, say a 10 degree bend, the wire may take a shape of an 8 degreebend due to this elastic property. Hence, some overbending of thearchwire may be needed to account for this elastic deformation.Solutions for overbending wire are provided. One method is a force-basedapproach. In this approach, the robot comprises a force sensor systemfor detecting forces generated by the wire after the wire has been bentby the first and second gripping tools. Depending on the direction andmagnitude of the detected forces, additional bends are formed in thewire. The proper bend in the wire is deemed to exist when the wire, atits designed shape, exhibits zero or substantially zero forces.

An alternative approach to overbending is based on deformation. In thisapproach, the wire is bent, the wire is released from the moveablegripping tool and a measurement is made of the wire's shape, the wire isbent some more (assuming more bending is required), the wire is releasedagain, and the process continues until the resulting configuration isthe one specified by the input file. In this embodiment, a camera orother optical technique can be used to measure the shape of the wire.Alternatively, force sensors can be used to determine the actual bend inthe wire (by moving the moveable gripper holding the wire to theposition where no forces are measured), and a measurement is taken toindicate what additional bends, if any, are needed to result in thedesired configuration.

It is further contemplated that a database of overbending informationcan be acquired as the robot bends wires. This database of overbendinginformation can be used by artificial intelligence programs to derive arelationship between overbending and desired bends, for a particulararchwire material. It may be possible to overbend wires in a singlestep, that is without requiring a lot of intermediate bending steps,based on this database of information, or based on a derivedrelationship between overbending and resulting wire shape.

In another aspect, the robot includes a heating system to apply heat tothe archwire or other workpiece while it is in the bent condition and/orduring bending. A current-based resistive heating system and heatedgrippers are used in the illustrated embodiment. This system allowsshape memory alloys to be bent by the robot and the acquired bendsretained in the wire material. Other heating systems are possibledepending on the nature of the device being bent.

In another aspect, the robot is part of an archwire manufacturing systemincluding a magazine containing a plurality of straight archwires. Themagazine holds the archwires such that they are spaced from each otherso as to enable the robot to grip an individual one of the archwires.Several different magazine designs are proposed. After the robot hasformed the archwire, the archwire is placed at a finish location. Aconveyor system carries the finished archwire from the finish locationto a labeling and packaging station. The wires are individually labeledand packaged. Alternatively, pairs of wires could be labeled ascorresponding to a single patient and packaged together.

In still another aspect, a gripping tool for a bending robot isprovided. The gripping tool includes a pair of opposing gripping fingersmoveable between open and closed positions, and a force system coupledto the gripping fingers for detecting forces imparted by a workpiecesuch as an archwire or other medical device after a bend has been placedin the workpiece. As noted above, the force system can be used tomeasure resulting forces after a certain bend has been placed in thewire, and the measurements used to indicate additional bending steps toyield the required configuration taking into account the need foroverbending.

In still another aspect of the invention, a method is provided forbending an orthodontic archwire in a bending robot. The method includesthe steps of

a) gripping the archwire with a first gripping tool such that a portionof the archwire projects beyond the first gripping tool;

b) gripping the portion of the archwire extending beyond the firstgripping tool with a moveable gripping tool;

c) releasing the gripping of the archwire by the first gripping tool;

d) moving the moveable gripping tool while gripping the archwire so asto draw the archwire through the first gripping tool a predeterminedamount;

e) the first gripping tool again gripping said archwire after the stepof moving is performed, and

f) moving the moveable gripping tool relative to the first gripping toolso as to place a bend in the archwire having a desired configuration.

In the above method, the moveable gripping tool and first gripping toolcan cooperate to place a series of bends in the archwire. It has beenfound that the movement called for by step f) should be performed suchthat a constant distance, equal to the length of archwire pulled throughthe fixed gripping tool in step d) is maintained between the fixedgripping tool and the moveable gripping tool. This distance should bemaintained in order to avoid applying tension or compression to thewire. Since the moveable gripping tool is moving potential in threedimensions during the bending, the distance that needs to be maintainedis measured along the length of the archwire. The same principle holdstrue for bending other types of devices.

These and still other aspects of the invention will be more apparent inview of the following detailed description of a presently preferredembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of an orthodontic care system incorporating ahand-held scanner system and an archwire manufacturing system inaccordance with a representative embodiment of the invention. Thehand-held scanner is used by the orthodontist to acquirethree-dimensional information of the dentition and associated anatomicalstructures of a patient and provide a base of information to plantreatment for the patient. The archwire manufacturing system includes awire bending robot to manufacture customized orthodontic archwires forshipment to the clinic.

FIG. 2 is a schematic representation of an archwire manufacturing systemshown in FIG. 1; FIG. 2A is a block diagram of the system.

FIG. 3 is a perspective view of a moveable robot arm used in themanufacturing system in FIG. 2; in FIG. 3 the gripping tool at thedistal end of the arm is omitted for sake of clarity to show the variousother aspects of the arm.

FIG. 4 is perspective view of the robot arm of FIG. 3, showing themovement of one of the arm joints and the corresponding motion of thearm.

FIG. 5 is a detailed perspective view of the gripping tool that ismounted to the distal end of the moveable robot arm of FIG. 3 and FIG.4.

FIG. 6 is a perspective view of the fixed gripping tool of FIG. 2 andthe gripping tool of FIG. 5, in which an orthodontic archwire is grippedby the tools.

FIG. 7 is a perspective view of a magazine of FIG. 2 that holds aplurality of straight archwires.

FIG. 8 is a detailed perspective view of the moveable gripping toolgrasping one of the archwires from the magazine of FIG. 7.

FIG. 9 is a perspective view of an alternative arrangement of a moveablerobot arm.

FIGS. 10A and 10B are perspective views of alternative magazineconstructions to the magazine of FIG. 7.

FIG. 11 is a schematic illustration of a conveyor system that carriesarchwires to the robot arm of FIG. 2.

FIG. 12 is a diagram illustrating the robot software as it relates tothe production flow in producing orthodontic archwires.

FIG. 13 is a simplified illustration of a set of teeth showing theorigin of a coordinate system that is used to calculate bracket locationfor a set of brackets, in three dimensions, for a patient. The bracketlocation for the teeth in a target situation determines the shape of anorthodontic archwire.

FIG. 14 is an illustration showing the vectors drawn from the origin ofthe coordinate system to the center of the brackets.

FIG. 15 is a perspective view of an orthodontic bracket.

FIG. 16 is an illustration of a vector drawn from the origin of thecoordinate system to the bracket, a normal vector N perpendicular to theslot surface of the bracket, and a tangential vector T extending in thedirection of the slot of the bracket.

FIG. 17 shows the normal vector Y for a particular bracket, thetangential vector X, the tangential distance T_(d) and antitangentialdistance AT_(d).

FIG. 18 shows in matrix form the values for an individual bracket whichdescribe the location of the bracket and its orientation, which are usedto generate the commands for the robot to form the orthodontic archwire.

FIG. 19 is an illustration of a set of points P1, P2, P3, . . . PN whichrepresent a set of bending points associated with individual bracketsfor a patient in a target situation. The location of the points in thethree-dimensional coordinate system is known.

FIG. 20 is an illustration of a section of wire between points P1 and P4in which a bend is placed between points P2 and P3.

FIG. 20A is an illustration of four points and a curve defined by aBezier spline, a technique used to calculate the shape of the bend inthe wire between points P2 and P3 in FIG. 20.

FIG. 20B is a flow chart of an algorithm to calculate the Bezier splineof FIG. 20A and the length of the curve.

FIG. 21 is a graph of force as a function of deflection for a workpiecesuch as a wire. The graph illustrates that that when a certain amount offorce, F1, is applied to the workpiece and then released, a deflectionD2 results. When the force is released, the amount of remainingdeflection, D1, is less than the deflection observed when the force isapplied to the wire, D2, since the wire has elastic properties.

FIG. 22 is an illustration of the stresses found in a wire when it isbent.

FIG. 23 is an elevational view of the gripping fingers of the fixedgripping tool of FIG. 6, showing the origin of a coordinate system usedby the robot in bending wire.

FIG. 24 is a top view of the gripping fingers of FIG. 23.

FIG. 25 is flowchart illustrating a deformation-controlled overbendingprocedure, which may be used to compensate for the elastic properties ofthe wire demonstrated by FIG. 21.

FIG. 26 is an illustration showing the overbending method set forth inFIG. 25.

FIGS. 27A-27E are a series of schematic drawings of the fixed andmoveable gripping tools of FIG. 6, showing how they moved relative toeach other and grip and release the archwire to place the archwire inposition to form a bend between points P2 and P3 of FIG. 19.

FIG. 28 is a schematic illustration showing how the movable grippingtool bends an archwire while maintaining a constant distance from thefixed gripping tool.

FIGS. 29A-29C illustrate how a bend may be formed in a series of steps.

FIGS. 30A-30D are a series of schematic drawings of the fixed andmoveable gripping tools of FIG. 6, showing how they move relative toeach other to place the archwire in position to form a bend betweenpoints P4 and P5.

FIG. 31 is an illustration of the points defining a portion of anarchwire, and illustrating a technique for placing bends in wires wherea substantial distance exists between the straight wire segments.

FIG. 32 is an illustration of a portion of an archwire showing a bendformed therein to increase the forces applied by the wire when the wirehas nearly straightened out, e.g., near the end of treatment.

FIG. 33 shows the wire segment of FIG. 32 installed between two teeth.

FIGS. 34 shows a wire with a loop in the wire.

FIGS. 35A-35B illustrate one possible method of forming the loop in thewire of FIG. 34.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Part 1. Overview

FIG. 1 is an illustration of an orthodontic care system 10 incorporatinga hand-held scanner system 12. The scanner system 12 includes ahand-held scanner 14 that is used by the orthodontist to acquirethree-dimensional information of the dentition and associated anatomicalstructures of a patient. The images are processed in a scanning node orworkstation 16 having a central processing unit, such as ageneral-purpose computer. The scanning node 16, either alone or incombination with a back-office server 28, generates a three-dimensionalcomputer model 18 of the dentition and provides the orthodontist with abase of information to plan treatment for the patient. The model 18 isdisplayed to the user on a monitor 20 connected to the scanning node 16.

The orthodontic care system consists of a plurality of orthodonticclinics 22 which are linked via the Internet or other suitablecommunications medium 24 (such as the public switched telephone network,cable network, etc.) to a precision appliance service center 26. Eachclinic 22 has a back office server work station 28 having its own userinterface, including a monitor 30. The back office server 28 executes anorthodontic treatment planning software program. The software obtainsthe three-dimensional digital data of the patient's teeth from thescanning node and displays the model 18 for the orthodontist. Thetreatment planning software includes features to enable the orthodontistto manipulate the model 18 to plan treatment for the patient. Forexample, the orthodontist can select an archform for the teeth andmanipulate individual tooth positions relative to the archform to arriveat a desired or target situation for the patient. The software moves thevirtual teeth in accordance with the selections of the orthodontist. Thesoftware also allows the orthodontist to selectively place virtualbrackets on the tooth models and design a customized archwire for thepatient given the selected bracket position. When the orthodontist hasfinished designing the orthodontic appliance for the patient, digitalinformation regarding the patient, the malocclusion, and a desiredtreatment plan for the patient are sent over the communications mediumto the appliance service center 26. A customized orthodontic archwireand a device for placement of the brackets on the teeth at the selectedlocation is manufactured at the service center and shipped to the clinic22.

As shown in FIG. 1, the precision appliance service center 26 includes acentral server 32, an archwire manufacturing system 34 and a bracketplacement manufacturing system 36. The details of the scanning systemper se are not particularly relevant to the wire bending features of theinvention and are therefore omitted from the present discussion for sakeof brevity except to the extent relevant to the present invention. Formore details on these aspects of the illustrated orthodontic caresystem, the interested reader is directed to the patent application ofRudger Rubbert et al., filed Apr. 13, 2001, entitled INTERACTIVE ANDARCH WIRE-BASED ORTHODONTIC CARE SYSTEM BASED ON INTRA-ORAL SCANNING OFTEETH, Ser. No. 09/835,039, now U.S. Pat. No. 6,648,640, the contents ofwhich are incorporated by reference herein, and the patent applicationof Rüdger Rubbert et al., SCANNING SYSTEM AND CALIBRATION METHOD FORCAPTURING PRECISE THREE-DIMENSIONAL INFORMATION OF OBJECTS, Ser. No.09/834,593, also filed on Apr. 13, 2001, the contents of which areincorporated by reference herein.

Part 2. Archwire Manufacturing System

A. Robot Design

FIG. 2 is a schematic representation of a presently preferred archwiremanufacturing system 34 shown in FIG. 1. The key aspects of the system34 are shown in block diagram form in FIG. 2A. The system 34 includes acontrol system for controlling the operation of a wire bending robot604. The control system in the illustrated embodiment comprises ageneral purpose computer 600 running a bending program and a robotcontroller 602. The computer 600 receives an input file from theprecision appliance service center computer that contains information asto the location of bracket slots in three dimensions, when the teeth arein a target situation. The computer 600 supplies the robot controller602 with wire position information corresponding to points along thewire where bends need to be made. This information is translated by thecontroller 602 into motion commands for the wire bending robot 604.

It will be appreciated that the system works in an analogous fashionwhen bending other types of medical devices. The computer 600 receivesan input file from some source that provides information as to how themedical device in question needs to be bent. The computer 600 suppliesthe robot controller 602 with position information corresponding topoints along the length of the medical device where bends need to bemade, and the robot responsively bends a medical device in accordancewith the input file.

The wire bending robot 604 consists of a moveable arm 606 having agripping tool at the distal end thereof. The moveable arm has a proximalend mounted to a table or base 610. The robot also includes a firstgripping tool 608. In the illustrated embodiment, the first grippingtool 608 is fixed with respect to the base 610 and mounted to the table.Hence, the first gripper tool 608 will be referred to hereinoccasionally as the “fixed gripper.” It would be possible to place thefirst gripper tool 608 at the end of second robot arm, in which case thefirst gripper tool would not necessarily be fixed, but rather would befree to move in space relative to the source of archwires, or the othermoveable arm. In such as system, a coordinate system would be definedhaving an origin at which the first tool is positioned at a knownposition. The bending commands for the robot would be with respect tothis known point.

A wire or other workpiece to be bent is held by the first gripper 608and the gripping tool at the end of the moveable arm 606, and the arm ismoved to a new position in space to thereby bend the workpiece. Thedetails of the construction of the arm 606 and fixed gripper 608 aredescribed in further detail below.

The system 34 of FIG. 2 is set up to manufacture customized archwiresone after the other continuously, as would be case in a precisionappliance service center serving a plurality of clinics. As such, therobot 604 includes a source of archwire material. The source could be aspool of wire in which case a cutting tool is needed to cut lengths ofwire for individual archwires. Alternatively, as shown in FIG. 2 and 7the source consists of a magazine 614 containing a plurality of straightarchwires 615 ready to be grasped by the gripping tool at the end of themoveable arm 606. In an embodiment in which the first gripping tool ismounted to the end of a moveable arm, the first gripping tool couldretrieve the next workpiece while the moveable arm places the finishedworkpiece at an exit location.

After an archwire is bent in accordance with an input file supplied tothe computer 600, the moveable gripping tool at the end of the robot arm606 places the wire (or workpiece being bent) at an exit locationindicated at 616. A conveyor system 618 including a plurality of trays620 is provided for carrying the finished archwires wires 622 from theexit location 616 to a labeling and packaging station 624. The labelingand packaging station 624 includes a printer 626 that prints a label forthe archwire and a magazine 628 containing a supply of packages such asboxes or bags for the wires. A worker at the station 624 takes a labelfrom the printer and applies it to the archwire 622 or to the packagefor the wire. The conveyor system 618 is also based on a commerciallyavailable, off-the-shelf conveyor system, such as of the type availablefrom the Montech division of Montrac.

The wire manufacturing system 34 includes a heating controller 612responsive to commands and settings from the wire manufacturing computer600. The controller 612 controls the supply of current to heatingelements 607A and 607B in the gripping fingers in the gripping tools inthe robot, to thereby heat the gripping fingers above ambienttemperature. Temperature sensors 605A and 605B detect the temperature ofthe gripper fingers and are used for feedback control of the heating ofthe gripper fingers. A direct or indirect system for measuring thetemperature of the workpiece may also be provided, such as infrared heatdetector. The heating controller 612 also controls a wire heating powersupply 611 that supplies a current to the gripping tools when they arebending a wire. The power supply 611 is used when the robot is bendingshape memory materials or Titanium Molybdenum Alloys (TMA) materials, orpossibly other materials. The current produces a resistive heating inthe wire. The current is controlled via a wire heating current sensor613 so as to produce a wire temperature at which a bend formed in thewire is set into the material. The heating of the gripping fingersavoids excessive heat loss when resistive heating of the wire isperformed.

FIG. 3 is a perspective view of a moveable robot arm 606 used in themanufacturing system in FIG. 2. In FIG. 3, the gripping tool at thedistal end 634 of the arm is omitted. In a preferred embodiment, themoveable arm is based on an off-the-shelf six-axis robot arm and fixedtool. A suitable arm, fixed tool, and robot controller for the robot 604is available from Stäubli Unimation of Germany. The Stäubli robot armand fixed tool is customized with gripping fingers, heating controllerand ancillary systems, software, current heating subsystem, forcesensors, and other features as described herein for bending archwires orother suitable medical devices.

The arm 606 consists of a proximal end or base 630 which mounts to thetable 610 of FIG. 2 and a free distal end 632 consisting of a toolflange 634, where the second gripping tool 651A of FIG. 6 is mounted, asdescribed below. The arm 606 is capable of motion along six differentrotational axes, with the directions indicated by the arrows numbered1-6 in FIG. 3. Thus, the base is fixed with respect to the table 610 andthe head portion 636 rotates relative to the base 630. A joint in headportion 636 rotates arm segment 638 about an axis indicated by arrow 2.Similarly, the arm segment 640 is rotated about an axis indicated byarrow 3 by a joint 639. A joint 642 rotates an arm segment 640 about theaxis indicated by the arrow 4. A joint 644 attached to the end of armsegment 640 rotates the tool flange 634 about the axis indicated byarrow 5. A sixth joint (not shown) rotates the tool flange 634 about anaxis indicated by arrow 6.

FIG. 4 is perspective view of the robot arm of FIG. 3, showing themovement of the arm joint 639 and the corresponding motion of the armsegment 640 and tool flange 634. The motion commands for the robot aresupplied from the robot controller 602 along a cable 650 which plugsinto the base 630 of the robot.

FIG. 5 is a detailed perspective view of a preferred gripping tool 651Athat is mounted to the tool flange 634 at the distal end of the robotarm 606 of FIG. 3 and FIG. 4. The construction shown in FIG. 5 alsoapplies to the fixed gripper 608 of FIG. 2 and FIG. 6. The gripping tool651A consists of a pair of opposed gripping fingers 652 and 654 whichopen and close to grip and release the workpiece, in this case anorthodontic archwire 615. A pneumatic cylinder actuates gripping finger652 by moving it about an axis relative to finger 654, to thereby permitthe fingers 652 and 654 to grip and release a workpiece such as a wire.A positioning sensor 655 (FIG. 2A) detects the position of the fingers.

In a representative embodiment, the archwires are made from a shapememory alloy such as Nitinol, a material based on Nickel and Titaniumplus Copper or other alloy. These materials can be heated to retain theshape of a bend formed in the wire. Accordingly, the wire heating powersupply 611 of FIG. 2A supplies a current to the gripping fingers of thefixed gripping tool and the moveable gripping tool. The flow of currentalong the wire creates a resistive heating of the wire sufficient forthe material to take a set according to the shape of the wire as it isbent. To avoid dissipation of heat from the wire into the grippingfingers, the gripping fingers 652 and 654 are preferably heated byelectrical heating elements 607A and 607B. Conductors 660 and 662 supplycurrent to the heating elements in the gripper fingers.

The gripping tool 651A of FIG. 5 further includes a force sensor 664 inthe form of a strain gauge. The force sensor is designed to detectforces that the wire imparts to the gripping fingers after a bend hasbeen placed in the wire or during the bending movement. The forcesdetected by the force sensor 664 are determined both in magnitude and indirection in three-dimensions. The use of the output from the forcesensors to overbend wire is explained in further detail below.

FIG. 6 is a perspective view of the fixed gripper 608 having a grippertool 651B as shown in FIG. 5 along with a moveable gripping tool 651Alocated at the distal end 632 of the moveable arm 606. An orthodonticarchwire 615 is shown gripped by the gripping tools 651A and 651B. Thefixed gripper 608 and gripping tool 651B remain stationary relative tothe base 610. The archwire is bent or twisted by the fixed gripping tool651 grasping the archwire 615 and the moveable gripping tool 651A alsograsping the archwire 615, and then moveable gripping tooling 651Abending the wire by moving to a new location in three-dimensional spacerelative to the fixed gripping tools. The location in space for themoveable arm to move to is determined by the input file fed to the robotcomputer 600. Basically, the input file consists of a series of pointlocations in a three dimensional coordinate system which correspond tobracket locations and orientation in a three-dimensional coordinatesystem for the arch, as described in more detail below. The manner ofcalculation of these points and generating movement commands (i.e., armposition and switch states for the gripper fingers) for the robot'smoveable arm and commands for the fixed gripper to bend the wire will bedescribed in further detail below.

Other possibilities exist for input files and calculation of the bendingpoints. For example, in extraction cases, the wire is needed to close agap between teeth and the wire serves as a guide or rail for the bracketto slide along to close the teeth. In this situation, a smooth curve isneeded between the teeth to allow the brackets to slide the requiredamount. In this situation, the space between the teeth is divided intosmall sections, and wire coordinates are obtained for each section. Aseries of small bends are formed at each section to generate therequired smooth curve. It may be helpful in this situation to round theedges of the gripping fingers to help provide the desired smooth shape.As another alternative, free-form curves can be formed by bending thewire between two points which would encompass a plurality of brackets.

While the preferred embodiment of a robot arm is shown in FIG. 3, thatis not the only possible arrangement of a robot arm. The robot of FIG. 3is optimized for complex bends and twists in archwires. However, somemedical devices or archwires may need only simple bends, in which case alesser number of joints may be required. For example, a one, two orthree axis robot may be sufficient for some applications.

FIG. 9 is a perspective view of an alternative arrangement of a six-axismoveable robot arm. In this embodiment, the robot arm comprises a base700, a motor that moves arm segment 702 along direction 704, a secondsection that contains a motor that moves second arm segment 706 alongdirection 708, and a third section 710 that contains a motor moving thearm section 712 along direction 714. A motor is provided for rotatingarm section 712 about axis α. A section 716 is connected to the end ofsection 712 and includes a motor for rotation of section 716 about anaxis indicated by β. A sixth section 718 is rotated about an axisindicated by γ. The gripping tool 651 A is mounted to the end of thesection 718. Robots of this type are also known and suitable for formingthe basis of an orthodontic archwire bending robot. The term “moveablearm” as used in the claims is intended to interpreted broadly toencompass the arm segments of the type shown in FIG. 9 as well as theconstruction shown in FIG. 4.

The gripping fingers of the gripping tools 651A and 652 preferablyoptimized, in terms of their physical configuration, for the type andshape of the workpiece being bent. This shape may change depending onthe nature of the workpiece, e.g., wire, fixation plate, spectacleframes, etc. In the case of wires, wires come in various cross-sectionsand sizes. It may be desirable to form a plurality of contours in thegripping fingers so as to enable the gripping fingers to grip severaldifferent types and sizes of wires without changing gripping tools. Forexample, one part of the gripper fingers has a series of rectangularcontours to grip wires of rectangular cross-section of varying sizes,and perhaps one or more circular contours to grip round wires.

The force sensors on the gripping tools may also be used to providefeedback for an adjustable gripping force to be applied to the workpiece(e.g., wires). It may be desirable to allow the wire to slide throughthe gripper fingers if the forces acting from the workpiece to thegripper exceed a certain limit. When these forces are sensed, the fixedgripper loosens its grip on the workpiece and allows it to slide.

FIG. 7 is a perspective view of a magazine 614 of FIG. 2 that holds aplurality of straight archwires needing to be bent in an presentlypreferred embodiment. FIG. 8 is a detailed perspective view of themoveable gripping tool grasping one of the archwires from the magazineof FIG. 7.

The magazine 614 consists of a tray 670 having a set of parallel raisedelements 672 that define a series of grooves 674 in the upper surfacethereof. The archwires 615 are placed in the grooves 674. The archwiresare maintained spaced apart from each other in the tray. This permitsthe robot's moveable gripping tool 651A to pick up a single archwire ata time from the magazine 614 as shown in FIG. 8 and insert it into thefixed gripping tool to commence a bending operation. Also, the magazine614 is positioned at a known location and the dimensions of the tray andslot features thereof are known precisely. This location information issupplied to the robot control software and allows the gripping tool 651Ato remove the archwires one at a time from the magazine automaticallyand without human assistance. When the magazine 614 is empty a full oneis placed at the same location.

FIGS. 10A and 10B are perspective views of alternative magazineconstructions to the magazine of FIG. 7. In FIG. 10A, the magazine 614consists of a cylindrical holder with a plurality of apertures 680spaced from each other, each containing a straight archwire 615. In FIG.10B, the archwires are in a rectangular holder with the aperturesarranged in rows and columns. In either case, the moveable arm grips anindividual one of the archwires and removes it from the magazine byvirtue of the spacing of the wires from each other in the magazine andbecause the location of each wire in the magazine can be known.

FIG. 11 is a schematic illustration of a conveyor system 722 including aconveyor belt 724 that carries archwires 615 to the robot. Here, therobot is enclosed within a safety cage 726. A source feeds archwires 615into slots 728 in the conveyor belt. When a wire has been bent and therobot is ready for a new wire, the belt advances one position and therobot grips the next wire placed on the belt 724. As anotheralternative, a spool of archwire can be fed to the robot and a cuttingtool (not shown) provided for cutting the wire from the spool into adesired length. The cutting tool could be incorporated into the end of asecond robot arm, for example. Still further implementations arepossible.

It also possible for the archwire manufacturing system to have otherworkstations or workplaces in which one or more of the following tasksmay be performed: loop bending, surface refining, and marking of thewires. These stations could be positioned at locations around theconveyor system 722 or be in separate locations.

It is also possible to enclosed the robotic wire bending system withinan enclosure and fill the enclosure with an inert gas such as nitrogen.The inert gas prevents oxidation of the workpiece during bending oroxidation or other chemical reaction affecting the gripping tools.

Appliance Manufacturing

The production flow for manufacturing archwires (or other similarappliances) with a representative embodiment of the wire manufacturingsystem of FIG. 2 is shown in FIG. 12. The production flow includes thestep 800 of loading a wire magazine 614, such as spool of wire in analternative embodiment, feeding the wire to the robot at step 802 andcutting the wire to length at step 804. At step 806, a series of bendsare placed in the archwire in accordance with the prescription for thearchwire. After the bending is complete, the wires are labeled at thestation 624 at step 808 and packaged in a box or other package at step810.

The bending of the wire at step 806 is based on slot data for bracketslots at described below in conjunction with FIGS. 13-20, or based onsome other suitable criteria as explained herein. The wire bendingcomputer 600 receives this slot data from the precision appliance centercomputer of FIG. 1. The computer 600 executes a bending program thatprocesses the slot data into a set of points in three dimensional spaceand calculates movements of the moveable arm necessary to achieve theappropriate bends in the wire. The computer 600 has a software interface812 to the robot controller, which translates position or movementsignals for the robot arm into low level instructions for the robotcontroller 602. The robot controller executes a robot control program(adapted from the control program that comes with the robot) whichcauses the robot arm 606 to move relative to the fixed gripper 608 tobend and/or twist the wire. Where the archwire is a shape memory alloy,the wire heating power supply 611 supplies current to the gripperfingers 652 and 652 on the moveable arm and the gripper fingers on thefixed gripper 608 to heat the wire while the wire is held in the bentcondition, and/or during bending motion, to set the shape of the wire.

Robot Input File

The input file, which dictates the shape of an archwire after bending,will now be discussed in conjunction with FIGS. 13-20. The input fileincludes a set of matrices, one matrix for each bracket in the arch ofthe patient. Each matrix consists of a combination of a vector oflocation of a point on the bracket and a matrix of orientation,indicating the orientation of the bracket in three-dimensional space.Both the vector of location and the matrix of orientation are based onthe position of the brackets on the teeth when the teeth are in a targetsituation. The target situation is developed by the orthodontist fromthe scan of the dentition and the execution of a treatment planningusing the treatment planning software at the clinic.

FIG. 13 illustrates the target situation for one arch 820 a patient. Thetarget situation is a three dimensional virtual model of the teeth 822in which virtual brackets 824 are placed, for example, on the labialsurface of the teeth. A coordinate system is defined for the arch 820having an origin 826. The coordinate system is in three dimensions, withthe X and Y dimensions lying in the plane of the arch and the Zdirection pointing out of the page. The location of the origin 826 isnot particularly important. In the illustrated embodiment, an average“mass” is assigned to each virtual tooth in the arch, and a center of“mass” is calculated for the arch 820 and the original 826 is located atthat center.

As shown in FIGS. 14 and 15, a vector of location 828 is defined foreach bracket. The vector 828 extends from the origin 826 to the centerof the slot 830 in the bracket along the wall 832 of the bracket slot,i.e., to point 834. The vector of location consists of the X, Y and Zcoordinates of the point 834 in the defined arch coordinate system.

The orientation matrix consists of a 3×3 matrix of unit vectors of theform: $\begin{matrix}\begin{matrix}X_{1} & Y_{1} & Z_{1} \\X_{2} & Y_{2} & Z_{2} \\X_{3} & Y_{3} & Z_{3}\end{matrix} & \left. 1 \right)\end{matrix}$

where X₁ X₂ and X₃ are the X Y and Z components of the X unit vectorshown in FIG. 15, Y₁ Y₂ and Y₃ are the X, Y and Z components of the Yunit vector shown in FIG. 15, and Z₁ Z₂ Z₃ are the X, Y and Z componentsof the Z unit vector shown in FIG. 15. As noted above, the matrix foreach bracket thus consists of the combination of the 3×3 orientationmatrix and the position matrix, and is thus as follows: $\begin{matrix}\begin{matrix}X_{1} & Y_{1} & Z_{1} & X \\X_{2} & Y_{2} & Z_{2} & Y \\X_{3} & Y_{3} & Z_{3} & Z \\0 & 0 & 0 & 1\end{matrix} & \left. 2 \right)\end{matrix}$

where X, Y and Z in the right hand column of entries is the positionvector.

The robot input file also includes an antitangential value and atangential value for each bracket. The antitangential value consists ofthe distance from the center of the bracket slot (point 834) to a pointdefining the terminus of the previous bend in the wire. The tangentialvalue consists of the distance from the center of the bracket slot tothe point defining the terminus of the next bend in the wire. The inputfile also consists of the thickness of the wire, as measured in thedirection of the Y unit vector in FIG. 15.

With reference to FIG. 16, an example of the 4×4 matrix 2) for a rearmost molar of a patient will be described. Figure shows the origin 826,the position vector 828, and the X and Y unit vectors which indicate theorientation of the bracket slot. FIG. 16 also shows the scale (in unitsof millimeters) which gives absolute location and orientationinformation for the bracket slot. Here, we assume in the example thatthere is no Z component to the tangential vector X or the normal vectorY. FIG. 17 shows the tangential distance T_(D) and the antitangentialdistance AT_(D) as measured along the centerline of the archwire. Theresulting matrix is shown in FIG. 18.

From a set of the matrices as shown in FIG. 18 comprising all thebrackets in the arch, the robot bending program extracts a series ofline segments in three dimensional space, which are defined by theterminus of the antitangential and tangential distances for each bracketslot. The set of line segments 840 is shown in FIG. 19. The linesegments are defined by a set of points P1, P2, P3 . . . Pn having knownthree dimensional coordinates due to the known location of the bracketslots and the known tangential and antitangential distances. The linesegments can also be defined as a set of vectors having a location forthe head of the vector and a magnitude and orientation in threedirections. The following discussion will use the set of points P1, P2,P3 . . . PN. In FIG. 19, the slashes 842 indicate the end points of thebracket slot 830 of FIG. 15.

The bends need to be placed in the wire before point P1, between pointsP2 and P3, between points P4 and P5, etc., that is, between the bracketslots. The slot-to-slot bends of the complete archwire are bent sectionby section. To form one slot-to-slot bend, the wire is fed so that thefixed gripper tool 651B and the robot arm gripper tool 651A can grip thewire in its initial shape. The wire length between fixed gripper androbot arm gripper is equal to the curved length of the wire along thebend. The straight wire sections 840 between the bends have to fit tothe bracket slots. To bend the wire into the demanded shape, the maincontrol computer 600 sends signals to the robot controller 602. Therobot controller 602 generates signals to move the robot arm 606 withthe robot gripper tool 651A into a new position. The movement path isdefined by a bending trajectory. The bend is indicated at 844 in FIG.20.

To form one slot-to-slot bend (e.g., bend 844 between P2 and P3), theremight be several of these bending movements necessary. One slot-to-slotbend is considered finished if two consecutive straight wire sections(e.g., between P1 and P2 and between P3 and P4), have the desiredrelative positions between one another.

To achieve this position, there are different approaches dependent onthe wire material properties possible: a) bending material withelastic/plastic properties, such as stainless steel, b) bending materialwith shape memory properties, and c) bending TMA alloys.

Material with elastic/plastic properties must be overbent to compensatefor the elastic part of the deformation. The overbend process, which isdescribed in further detail below, can be defined as a closed loopcontrol. Within the first bending step, the robot arm 606 moves to a newposition. Preferably the new position is equal to the planned positionor to the planned position plus an amount of overbending. At the end ofthe move the forces and moments acting on the grippers are measured.They indicate the remaining elastic deformation in the wire. Todetermine the gripper position which correspond to the released wireshape, the robot arm 606 starts a new move in direction opposite to theacting forces and moments. The forces correspond to a translationalmove, the moments to a rotational move. By adjusting continuously themovement direction to the measured forces and moments, the robotachieves a position, where the forces and moments are in the order ofthe measurement resolution (zero-force-position). By choosing anappropriate measurement resolution, the remaining elastic deformationcan be neglected and the relative position of the two gripperscorresponds to the relative position of the straight wire sections inthe released situation. This zero-force-position is compared to theplanned position. If the differences are bigger than the tolerancelimits, an additional bending step follows to decrease the difference.From the zero-force-position the robot moves now in direction to theplanned position and overrides the planned position about the value ofthe difference between zero-force and planned position. The endpoint ofthis move is called overbend position. From the overbend position startsagain the force and moment controlled move to find the newzero-force-position. If the new zero-force-position is within tolerancelimits to the planned position, then the bending process for oneslot-to-slot bend is completed and the wire is fed to bend the nextslot-to-slot section. If the amount of overbend was too much, the newoverbend position is calculated as-described above. If the amount ofoverbend was not sufficient, then the new overbend position iscalculated as the former overbend position plus the difference betweennew zero-force-position and planned position. The described process isrepeated within a loop up to the situation, that the difference betweenzero-force-position and planned position is smaller than the tolerancelimit.

Materials with shape memory properties and TMA will be bent to theplanned position. To transfer this position into the memory of thealloy, the wire section between the two grippers is heated to a certaintemperature for a certain time. The heating is possible e.g. byconductive resistance heating, laser, convection, radiation, or applyingwarm air or liquid to the material. Heating current and time must beappropriately adjusted to the respective alloy, the wire section lengthand the wire shape. To warm-up the wire, the wire heating can startalready during the bending movement to the planned position. To avoid aheat sink effect at the gripper fingers and to ensure that the completeinter-bracket section of the wire obtains the necessary heating, thegripper fingers 652, 654 (FIG. 5) or at least the contact areas ofgripper and wire are heated too. The grippers may be heated continuouslyduring the production process of the whole archwire. To compensate foran incomplete transition of the bending position to the alloy memory,there can be defined a certain amount of overbending.

In bending TMA materials, the material can be heated to a hightemperature where there is no springback, however when the materialcools, it retains its springback properties. The procedure for bendingsuch materials is as follows: 1) heat the gripper fingers; 2) bend thewire to the desired configuration; 3) heat the wire up to thetemperature where the springback tendency no longer exists; 4) turn offthe heat source and allow the wire to cool, and 5) advance the wire tothe next position for bending; and then repeat steps 1)-5).

The bending of the wire from one section to the next requires that thelocation and alignment of one straight wire section (i), for example P3to P4, is defined in reference to the previous straight wire section(i−1) in the counting order defined in the bending system. The origin ofthe bending system is defined at the end of the straight wire section(i−1), which aims towards the following straight section (i). The x-axisis equal to the direction of the straight wire section (i−1) directed tosection (i). For wires with rectangular cross-section the y-axis isperpendicular to x and in direction to the wider dimension of the wire.For quadratic or circular cross-section the y-axis must be perpendicularto x and can be chosen according to practical reasons. The x,y,z-axisfollow the right hand rule.

The bracket slot data as described above needs to be transformed tobending data for use by the robot controller 602. This is done bycalculation, from the position of the bracket center point 834 (FIG.14), to the position of the straight wire section center point (locatedalong the middle of the wire, point 840 in FIG. 19).

Next, there needs to be a calculation of the bent wire shape and length.In FIG. 30, this is shown as the shape of the wire between points P2 andP3. For each bending step, the robot gripper grips the wire in a certaindistance from the fixed gripper corresponding to the length of the wirebetween points P2 and P3. The wire section between the two grippers willbe bent. To minimize the bending forces and moments and to ensure thatthe bending forces and moments don't exceed limits, which may causedamage to the equipment or to the wire, the gripped wire length shouldbe approximately the “natural” length of the wire in its bent shape. Ifthe length is too short, the wire will be torn and there will be hightensional forces, if it's too long, the wire will tend to kink.

The robot computer 600 therefore calculates the approximate shape of thebent wire using an appropriate algorithm. One way is deriving a secondor third order curve representing the shape of the wire using numericaltechniques. Another would be using a regular spline algorithm. Ideally,there should be no sharp bends in the wire. In the illustratedembodiment, a Bezier spline algorithm is used. The algorithm gives ananalytical description of a smooth curve and generates a set of pointsalong the length of the curve. The length of the curve is obtained bysumming up the distance (in three dimensions) along each segment of thecurve. The separation distance between each point in the segments can beset arbitrarily and in the illustrated embodiment is 0.05 mm. Thealgorithm is as follows:

Input:

Centerpoint location and alignment of two neighbored straight wiresections (given as 4×4 matrice with local vector {right arrow over (l)},tangential vector {right arrow over (t)} normal vector {right arrow over(n)} and vertical vector {right arrow over (v)}) These matricescorrespond to the 4×4 matrix described earlier.

tangential and antitangential distances s_(t) and s_(at)

Bezier-distance b_(d) (empirical value)

The Bezier formula, as known by literature, is described by four pointsas shown in FIG. 20A.

The points of the spline curve are given by:

{right arrow over (P)}=(1−v)³ ·{right arrow over (P₁)}+(1−v)² ·v·{rightarrow over (P₂)}+(1−v)·v ²·{right arrow over (P₃)} +v ³·{right arrowover (P₄)}νε{0, . . . , 1}

Here it will be noted that the Bezier points P₁ to P₄ in FIG. 20A arenot necessarily the points P1-P4 of the wire segments shown in FIG. 19,but rather are the points used to calculate the Bezier spline length andshape.

To describe the curved wire shape between the straight wire sectionsfrom slot (i−1) to slot i, the Bezier points {right arrow over (P₁)},{right arrow over (P₂)}, {right arrow over (P₃)}, {right arrow over(P₄)} are calculated by:

{right arrow over (P₁)}= {right arrow over (l_(i−1))}+ s _(t,i−1)·{right arrow over (t_(i−1))}

{right arrow over (P₂)}= {right arrow over (l_(i−1))}+( s _(t,i−1) +b_(d))·{right arrow over (t_(i−1))}

{right arrow over (P₃)}= {right arrow over (l_(i))}−( s _(t,i) +b_(d))·{right arrow over (t_(i))}

{right arrow over (P₄)}= {right arrow over (l_(i))}− s _(at,i) ·{rightarrow over (t_(i))}

The wire length L and the N intermediate spline points can be calculatedby the algorithm shown in FIG. 20B.

The empirical value Bezier-distance b_(d) must be set to make thecalculated and actual bent wire shape tally. For orthodontic wires, agood assumption is b_(d)=1x . . . 2x the larger wire cross-sectiondimension.

The bending trajectory needs to be calculated for each bend. The bendingtrajectory is a number of positions of the moveable arm's gripper 651Ain relation to the fixed gripper 651B, which connect the start positionand the destination position of a bending movement. In general there aretranslational and rotational movement components from each bendingtrajectory position to the next.

For each bending trajectory position the calculated length of the curvedwire must be equal to the calculated length in the planned position. Toavoid kinking condition for the wire the movement can be divided intotwo parts:

1. Initial movement to steer the wire into a distinctly deformed shape.The initial movement is defined as a rotational transformation around anaxis through the endpoint of the fixed gripper perpendicular to theconnecting vector of the start position and the end position. This isindicated in FIG. 29A by the rotational motion of the moveable grippingtool 651B and movements a, b, c, and d.

2. Finish movement to destination position. The finish movement is agradual approach from the start position (or if there is an initialmovement from the end position of the initial movement) to thedestination position. The translational and rotational parts of thewhole bending movement are split up steadily to the individual bendingtrajectory positions. Between two bending trajectory positions, themovement path of the robot gripper is defined by a linear translationalmovement along the straight line connection of the two positions and bysteadily divided rotational movement. The distance between two bendingtrajectory positions must be small enough to avoid kinking of the wireand inadmissible forces and moments. These movements are indicated atpositions d, e, f, and g in FIG. 29B.

In bending wire as described herein, the robot system 604 of FIG. 2 hasa coordinate system in which an origin 860 is defined as shown in FIGS.23 and 24. The wire passes through the fixed gripping tool 651B througha small gap formed when the fingers are in an open position. The origin860 is defined as the along the center of the axis of the wire at theplanar edge of the fixed gripping tool 651B. The robot controllersoftware knows the location of the moveable gripping tool relative tothe fixed gripping tool in this coordinate system at all times.Furthermore, the wire is gripped by the moveable gripping tool at aprecise point on the moveable gripping tool. Therefore, when the wire isheld by the fixed gripping tool and the moveable gripping tool, thedistance between the two is known exactly. This allows the bending shownin FIGS. 29A-29C to be executed without stretching or contracting thewire. In particular, the distance as measured along the wire between thefixed and moveable gripping tools at the point of attachment to the wireis constantly maintained a distance equal to the calculated Bezierdistance for the wire as bent between the points P2 and P3 of FIGS. 19and 20, and of course for subsequent bends.

To advance the wire between bends or to place the wire in condition forthe first bend, there are at least two possibilities. One is that themoveable gripper tool grips the wire and pulls it through the fixedgripping tool (with the fixed gripping tool opened to allow the slidingof the wire with respect to the gripping tool). As an alternative, thewire could be on a spool or coil, and the spool rotated by a motor toadvance the wire through the fixed gripping tool. In the laterembodiment, a cutting tool will need to be provided to cut the wireafter the bending is completed. Archwire manufacturers sell wires inbulk already cut to length and the present description is made in theembodiment in which the wire segment is advanced by the moveablegripping tool advancing the wire through the fixed gripping tool.

Having the bent wire between the two grippers in a tensed state, therobot gripper is moved to a new position, where no forces and momentsare acting on the gripper. The force sensors 640 on the fixed andmoveable gripping tools are used to determine the position. Thisposition is called the zero force position and corresponds to thereleased state of the wire. Forces, moments and the movements componentsare calculated in the main robot coordinate system of FIGS. 23 and 24.

Depending on the nature of the material, some overbending of the wiremay be needed. This would be indicated for example if the zero forceposition is not the same as the calculated position for the robot'smoveable arm 606. To better understand the overbending principles,attention is directed to FIGS. 21 and 22

FIG. 21 illustrates the relationship between force and deflection ofwire. The solid line 866 indicates how much force is needed to give acertain deflection of the wire. Where the force is less than F1, whenthe force is released the wire returns to its initial state andexperiences no deflection due to elastic properties of the wire. Atforce level F1, some permanent deformation, i.e., plastic deformation,of the wire starts to take place. With a force level of F2, the wire isdeflected an amount D2, but when the force is release from the wire thewire bends back to a deflection amount D1, with the relaxation curve 868essentially parallel to the curve 866 up to force level F1, as shown.Thus, some level of force indicated at F3 is required to be applied tothe wire such that when the force is removed the required deflection Dsis achieved in the wire. The fact that F3 is greater than F2 and thatthe deflection D3 is greater than D2 illustrates that some overbendingis generally needed to yield the proper shape of the wire after thebending forces are removed.

FIG. 22 illustrates the stresses involved with wire material when it isbent. The wire has one portion experiencing elongation stress, asindicated at 856, while compression stresses are experienced atlocations 858, with the length of the arrows indicating the relativemagnitude of the stress. With small bends, the forces at the center ofthe wire are small enough such that only elastic deformation occurs,while at the outside portion of the wire some plastic deformation mayoccur. The result is that bending wire is a mixture of elastic andplastic deformation, the character of which is determined by the amountof the bend, the wire material, and the cross-sectional shape.

To determine the amount of required overbending, there are severalpossibilities. One is a purely analytical solution like finite elementanalysis of wire. Alternatively, a piece of wire can be tested todetermine its response to known forces, and the result stored as acalibration table of bends. Basically, the curves in FIG. 21 areobtained experimentally. A more preferred approach uses force sensors640 (FIG. 2A) on the fixed and moveable gripping tools to sense the zeroforce position of the wire and compare the location of the moveablegripper in this position with the intended position. A geometrical ordeformation approach is another alternative. In this approach, the wireis bent some amount and then released, the relaxation position noted,the wire bent some more, the wire released, etc. the process continuinguntil the wire is bent to the desired position.

With a force based system, perhaps augmented by an adaptive,self-learning artificial intelligence type learning program orcalibration table based on previous bends of similar wire, the resultingconfiguration of the wire can usually be achieved more quickly.Basically, for every bend performed in the wire, information is storedas to the movement necessary to result in a specific bend. For example,to achieve a 13 degree bend in the wire of type T and cross-sectionalshape W, the wire had to be bent 15.5 degrees, and this information isstored. With enough data, a mathematical relationship can be derivedthat that represents curves 866 and 868 for the wire of type T (at leastin the portion of the curve of interest), and this mathematicalrelationship can be used, in conjunction with force sensors, to quicklyand accurately place the required bends in the wire.

In either situation, an optical system such as a camera could be usedfor detecting the position of the wire in the relaxed position is usedto determine the actual shape of the wire after any given bend.

FIG. 25 is a flow chart of a deformation controlled overbending processthat can be used with stainless steel or other wires, including shapememory wires where some overbending is still required.

At step 870, a calculation is made of overbending values in both atranslation and rotational aspect. This calculation could be performedfor example using finite elements methods, using a calibration table,using a derived mathematical relationship between force and bending,using stored values for overbending from previous bends, or somecombination of the above.

At step 872, the bending curve is calculated up to the planned positionand including the overbending values. This involves the Bezier splinealgorithm set forth previously.

At step 874, the moveable gripping tool is moved to the positionindicated by the sum of the planned position plus the overbendingposition. This forms a bend in the wire. Again, this position isdetermined in reference to the robot coordinate system and in referenceto the spatial relationship between the points where a bend needs to beplaced in the wire (P3 and P2 in FIG. 19, for example).

At step 876, the force sensors are used to measure the residual forcesimparted by the wire onto the gripping tools, and if the forces aregreater than some threshold, the moveable gripping tool 651A is moved tothe position where the force measurement is zero or substantially zero.

At step 878 the actual position of the moveable gripping tool is measureusing the position sensors in the moveable robot arm.

At step 880, a check is made to see if the difference between the actualposition and the planned position is less than a limit. If not, newoverbending values are calculated (step 882), and a new bending curve iscalculated to overbend the wire an additional, amount, in the desireddirection, to bring the wire closer to the desired shape (step 883).

Steps 874-883 are repeated until the difference between the actualposition of the moveable gripping tool and the planned position is lessthan a limit.

At step 884, the error in the actual position relative to the plannedposition is noted and compensated for by correcting the next slotposition. In particular, the next slot position represented by the nextpair of points in the set of points in FIG. 19 is translated in threedimensions by the amount of the error. This correction of the next slotposition is needed so as to avoid propagation of an error in any givenbend to all subsequent bends.

At step 886, the overbending results from steps 874-882 are saved inmemory and used in an adaptive technique for estimating futureoverbending requirements as explained above.

FIG. 26 is a schematic representation of the overbending performed bythe method of FIG. 25. The line 900 represents the slot between thefingers of the fixed gripper and the line 902 represents the wire invarious stages of bending. The dashed line 904 indicates the movement ofstep 874, with line 906 representing the slot of the moveable grippingtool between the gripping fingers where the wire is gripped in theplanned position. The zero force position where the moveable gripper ismoved to is indicated at 908 (step 876 in FIG. 25). There is both arotational (dw) and translational (dx) aspect to the difference betweenthe zero force position and the planned position. The last overbendposition is shown as position 910 (the overbend calculated at step 870).The required correction is shown by the position 912. This new overbendposition, calculated at step 880 in FIG. 25, is equal to the lastoverbend position 910 plus the planned position 906 minus the zeroposition 908. This new overbend position includes both translational androtational aspects as indicated by −dx and −dw.

One possible example of actual robot gripper movements to feed the wirethrough the grippers and execute a representative bend will be explainedconjunction with FIGS. 27A-28E. As shown in FIG. 27A, the wire is threadthrough the fixed gripper tool 651B or placed there by the moveablegripping tool such that some length of wire is extending beyond the endof the fixed gripping tool 651B. The points P1 and P2 along the wiresegment are indicated. The moveable gripper tool 651A is positionedabove and slightly to the right of the fixed gripping tool, here 0.2 mmaway. The moveable gripping fingers open and the moveable gripper movesdown to clamp the wire. The 0.2 mm distance is merely chosen so that thefingers to not collide and can vary from the illustrated embodiment. Thefingers cooperate with each other by translation movements of themoveable gripping tool and releasing and closing the fixed and moveablegrippers such that the wire is advanced through the fixed gripping tool.This is also indicated by FIG. 27B, showing the fixed gripping tool 651Bopen (OP) to allow the wire to be slid through a slot formed between thefingers of the tool 651B. The moveable gripping tool moves to the rightto draw the wire through the fixed gripping tool until the point P2 isto the right of the fixed gripping tool (as shown in FIG. 27C), where itcan be grasped by the moveable gripping tool. As shown in FIGS. 27C and27D, the moveable gripping tool opens and releases its grip on the wire(indicated by OP) and moves to the position where it closes (CL) andgrasps the wire at location P2. Then moveable gripping tool 651A drawsthe wire through the fixed gripping tool while gripping the wire atpoint P2, such that point P3 is located at the origin of the robotcoordinate system, as shown in FIG. 27D. Since the planned location ofboth P2 and P3 after a bend in the wire is made is known in the robotcoordinate system, the moveable gripping tool 651A moves to the positionshown in FIG. 27B to place a bend in the wire. At this point, if furtheroverbending is called for, the process of, e.g., FIGS. 25 and 26 isperformed to place the required overbend in the wire. The movements ofFIGS. 27B-27D could be combined to one movement if the distance is smallenough, and depending on the thickness of the wire.

FIG. 28 illustrates before and after positions of the wire when thebending of FIG. 27E occurs. The figure illustrates that the movement ofFIG. 27E is not a straight line movement which might cause excessiveelongation or kinking of the wire. Rather, the movement of the gripperis illustrated as steps a, b, c, d, e, f such that the distance, asmeasured along the length of the wire, is maintained constant. Themovement may performed in two stages (such as shown in FIGS. 29A and29B). The result is that two bends 912 are placed in the wire, as shownin FIG. 29C. Of course, a twist could also be performed in the wire ifrequired by the prescription.

After the bend has been placed in the wire, the steps shown in FIGS.30A-30D are performed to advance the wire along to the position of thenext bend. First, as indicated at FIG. 30A, the moveable gripping tool651A is translated to the right an amount indicated by ΔX. This movespoint P3 to the right of the fixed gripping tool by an amount ΔX, asshown in FIG. 30B. Then, the moveable gripping tool releases the wireand re-grips the wire to the right of the fixed gripping tool as shownin FIG. 30C and again translates to the right an amount sufficient tomove point P4 to the right of the fixed gripping tool. The moveablegripping tool releases the wire again and grips the wire at point P4.The wire is again translated to the right such that the situation shownin FIG. 30D is obtained. The wire is now in position for a bend in thewire between P4 and P5. The process of FIGS. 25 and 26 occurs again tocalculate new bending and overbending positions and the bend is formedin the wire. The process of FIGS. 30A-30D continues until all the bendshave been formed in the archwire. When the final bend is complete, thewire is released from the moveable gripper at the exit location of thewire manufacturing system, and carried by conveyor to the labeling andpackaging station described earlier.

Shape memory alloy materials require heating to take on the shape givenby the bend produced in FIG. 27E. Thus, for these wires, while the wireis held in the position shown by FIG. 27E, heat is applied to the wireto raise the temperature of wire to the value needed to take the set.The temperature varies with the type of material. In the illustratedembodiment, a resistance heating system is used as described previously.The current is adjusted until the proper temperature is reached. Theheat treating is deemed complete when the force sensors read zero (orless than some limit) when the wire is held by the grippers in theplanned position. The amount of current and time applied to the wire isagain stored information that can be used for future heating of the sametype of wire.

For some softer shape memory materials, e.g., NiTi, the force sensor 640(FIG. 2A) provided in the gripping tools must be very sensitive todetect the small forces involved. While shape memory materials may notrequire force sensors at all, they can be used to give information as tothe effectiveness of the heating process.

In a preferred embodiment, two force sensors are used. A coarser forcesensor, used for measuring larger forces during bending, is fitted tothe moveable gripping tool. A finer force sensor, with a higherresolution, low noise and higher sensitivity, e.g., with a sensitivityof less than 0.0005N, is fitted to the fixed gripping tool, in order todetect the zero force position. The force sensors are both based onstrain gauge technology and can be readily adapted from commerciallyavailable strain gauges or off the shelf units. For example, the finerforce sensor may have different amplifiers or other modifications to thecircuitry to have greater sensitivity, signal to noise ratio and forceresolution. Other types of force sensors, such as those based on piezotechnology, would be suitable. Suitable off-the-shelf strain gauge forcesensors are available from JR3 Inc. of Woodland Calif., model nos.45E15A-U760 (fixed gripping tool) and 67M25A-I40(moveable grippingtool).

Other types of heating systems could be adopted for archwires and othertypes of workpieces to be bent, such as laser, flame, infrared,conductive or radiant heating. Some springback may still be observed inshape memory materials even when heating is performed unless the wire isheated close to the maximum permitted temperature of the wire.Therefore, with some shape memory materials it may be desirable toperform some overbending in order to lower the temperature needed to setthe new shape into the wire. Again, the required amount of overbendingat a given wire temperature can be stored in memory and used to derive arelationship between temperature, overbending and resulting position forthe material, which can be used for subsequent bends in the wire.

Due to the complexities of wire deformation and twisting in wire thatcan occur when wire of a rectangular cross section is bent, and thedifficulty in controlling the resulting shape of the wire (particularlywhen complex bends and twists are formed in the wire), the usage offorce measuring devices, and position sensors to detect the shape of thewire when the wire is in a zero force condition, gives accurateinformation as to the shape of the wire after a bend. Thus, a forcebased approach to overbending is a preferred embodiment. The actualposition of the wire in the zero force condition can be obtained byposition sensors on the robot arm (which makes no contribution to themeasurement of forces), or better yet, by releasing the wire from themoveable arm and detecting the position of the wire with a camera orother optical system. Basically, the camera would image the wireimmediately in front of the fixed gripping tool. Pattern recognition andimage processing algorithms are used to determine the edge of the wire,and thereby calculate its shape. More than one camera could be used ifnecessary to image the wire sufficiently to calculate twist in the wire.The effects of gravity would have to be compensated for in any positionmeasuring system in which the wire is not held by the moveable grippingtool.

Thus, in one possible embodiment the robot further comprises an opticalsensor system such as a CCD camera detecting the shape of theorthodontic appliance after the bend in said orthodontic appliance hasbeen made, such as by releasing the appliance from the moveable grippingtool and allowing the appliance to take its natural position, and thenusing the optical system to detect the shape of the appliance.

It is also possible to use both the force measuring systems and theoptical system as a final check on the shape. The force sensor system(e.g., coupled to the fixed and/or moveable gripping tools) detectsforces generated by the orthodontic appliance after the orthodonticappliance has been bent. The moveable arm is operated to move theorthodontic appliance to a zero-force position in which the forcesdetected by the force system are below a predetermined threshold. Theoptical sensor system detects the shape of the orthodontic appliance inthe zero-force position. The position detected by the optical sensorsystem can be used as a feedback mechanism for further wire bending ifthe zero force position is not the intended or desired configuration ofthe appliance. An optional type of sensor system would be calipers thatcontact the workpiece and responsively provide position information asto the location (i.e., bend) formed in the workpiece.

For stainless steel wires, there is generally no need for heat treatmentof the wire. It is simply bent into the desired position, withoverbending performed as required. The shorter the distance betweenendpoints of a bend, the greater the deformation in the wire, thereforethe greater the predictability in the deformation. With orthodonticarchwires, the situation can occur where there is a relatively longdistance between bracket slots (particularly in the region of themolars) and it can be difficult to obtain a stable bending result. Apreferred solution here is to make this distance shorter by adding onsome length to the tangential distance of one slot position and theantitangential distance of the next slot position, as shown in FIG. 31.Here, point P2 is extended in space to point P2′, and point P3 isbrought closer to point P2 by moving it to point P3′. The requiredbending of the wire is now calculated relative to points P3′ and P2′.The bend placed between P3′ and P2′ is now sufficiently short that itcan be formed with enough control.

In practice, it known that after an archwire has been fitted to thepatient's brackets, the archwire imparts forces to move the teeth to thedesired position. However, after a certain amount of time, some smallamount of bend remains in the wire but it is insufficient to cause anyfurther tooth movement. Consequently, the teeth are not moved to theirdesired position. This can be compensated for by adding an additionalamount of bend to the wire so that when the wire is installed, it willcontinue to exert forces until the teeth have been moved all the way totheir desired position. As shown in FIG. 32, this small additional bendis shown as 920. FIG. 33 shows the wire of FIG. 32 installed in thebrackets of a patient. The bend 920 of FIG. 32 is in addition to otherbends that may be placed between the brackets 824. FIG. 33 illustratesthat enough residual force exists by virtue of bend 920 to move theteeth 822 to their desired position.

In certain orthodontic situations, loops may need to be bent in thewires. FIG. 34 illustrates a loop 922 formed in a wire 615. The loop maybe formed by the robot of FIG. 34. Alternatively, only the peripheralcorners 924 of the loop 920 are formed by the bending robot, as shown inFIG. 35A, with the remainder of the loop formed by placing the wire overa die 926 having a shape 928 matching the shape of the bottom portion930 of the loop 920. A forming tool 928 is moved against the wire anddie 926 to form the bottom portion of the loop as indicated in FIG. 35B.

The robot may also include a separate arm or tooling by which stops, orother features are bonded to the wire by suitable techniques such aswelding. For example, the robot can be fitted with other tools forforming a Herbst appliance or expansion devices. Alternatively,different types of wires could be joined together by welding.

The robot may also be used to bend clear, transparent wires made frompolymeric or plastic materials, such as thermoplastics, duroplastics,polyacrylic plastics, epoxy plastics, thermoplastics, fiber reinforcedcomposites, glass fiber containing materials or other similar materialssuitable for an orthodontic archwire. These plastics archwires mayrequire heating during bending, but current sources may not be suitableheating devices. Recommended techniques for heating the plastic wireinclude blowing hot air over the wires during bending, using heatedpliers, placing a heat conductive material onto the wire, using a laserto heat the wire, or spraying a hot vapor or liquid onto the wire.

As noted above, additional possibilities are presented for bendingfixation plates, orthotic devices, prosthetic devices, endodonticdevices, surgical guidewires, surgical archbars, implants or surgicaltools with the robot manufacturing system. The gripper fingers andassociated structures may be optimized depending on the workpiece orappliance in question. However, the principles of operation arebasically the same.

For example, the robot of the present invention is particularly usefulfor bending fixation plates, rods, compression plates and the like, forexample facial, cranial, spinal, hand, and long bone and otherosteosynthesis plates, such as, for example, the titanium appliancesprovided by Leibinger Gmbh of Germany. These fixation plates mayconsists of, for example, an elongate skeletal frame having a pluralityof apertures for receiving screws, arranged in straight lengths, C, Y, JH, T or other shape configurations, or a long cylindrical rod. At thepresent, these appliances are manually bent by the surgeon to the shapeof the bone in the operating room using special manual bending tools. Itis possible to automate this process and bend the plates in advanceusing the principles of the present invention. In particular, the shapeof the bone or bone fragments is obtained a CAT scan, from a scan of theexposed bone using a hand-held scanner (such as described in the patentapplication filed concurrently of Rudger Rubber et al. SCANNING SYSTEMAND CALIBRATION METHOD FOR CAPTURING PRECISE THREE-DIMENSIONALINFORMATION OF OBJECTS, Ser. No. 09/834,593, the contents of which areincorporated by reference herein. Once a three-dimensional virtual modelof the bone is obtained, e.g., from CAT scan data, the virtual model ismanipulated using a computer to fuse the bones together in the desiredposition. The surgeon then overlays the three-dimensional virtualimplant in the desired location on the virtual model, and bends thevirtual implant using the user interface of a general purpose computerstoring the virtual model of the bone and implant. The required shape ofthe implant to fit the bone in the desired location is derived.

Alternatively, a physical model of the bone in the desired configurationcan be manufactured from the virtual model using stereolithography(SLA), three-dimensional lithography, or other known technology, and theshape of the implant derived from the physical model.

As another alternative, a SLA physical model of the bones (e.g., skull)is made from a CT scan or other source, and the surgeon performs asimulated surgery on the physical model to place the bones in thedesired condition. The model is then scanned with an optical scanner anda virtual model of the bones in the desired condition is obtained, asdescribed in the patent application of Rudger Rubbert et al., citedabove. The virtual fixation device is then compared or fitted to thevirtual model of the bones to arrive at a desired shape of the fixationdevice.

In either situation, the shape of the implant is then translated to therobot controller as a series of straight sections and bends of knowngeometry (and specifically position commands for the moveable grippingtool relative to the fixed gripping tool). The moveable and fixedgripping tools of the bending device grip the implant or fixation deviceat one end and then either bend the appliance or advance the position ofthe implant to the location of the next bend, and continue along thelength of the device to form the device in the desired configuration.Obviously, some modification to the gripping tools may be needed fromthe disclosed embodiment depending on the physical characteristics ofthe device being bent, and such modifications are within the ability ofpersons skilled in the art.

The bending apparatus described above is also adaptable to genericworkpieces, such as tubes, cylinders, wires or other types ofstructures.

The bending apparatus may use resistive heating, force sensors,overbending, and the other features described at length in the contextof orthodontic archwires, depending on the application for otherworkpieces.

While presently preferred embodiments of the invention have beendescribed for purposes of illustration of the best mode contemplated bythe inventors for practicing the invention, wide variation from thedetails described herein is foreseen without departure from the spiritand scope of the invention. This true spirit and scope is to bedetermined by reference to the appended claims. The term “bend”, as usedin the claims, is interpreted to mean either a simple translationmovement of the workpiece in one direction or a twist (rotation) of theworkpiece, unless the context clearly indicates otherwise.

What is claimed is:
 1. A gripping tool for a bending apparatus, saidbending apparatus comprising a robotic arm moveable about multiple axes,comprising: a pair of opposing gripping fingers moveable between openand closed positions, said pair of opposed gripping fingers mounted tothe end of said robotic arm, said gripping fingers for gripping aworkpiece and thereby bending and/or twisting said workpiece by virtueof movement of said robotic arm while gripping said workpiece while afirst portion of said workpiece remains fixed relative to said grippingfingers; a force system coupled to said gripping fingers for detectingforces imparted by a workpiece either during bending or after a bend hasbeen placed in said workpiece; and a heating system for heating saidopposing gripping fingers above ambient temperature.
 2. The grippingtool of claim 1, further comprising a resistive heating System supplyinga current to said gripping tool so as to apply resistive heat to saidworkpiece.
 3. A gripping tool for a bending apparatus for bendingorthodontic archwires, said bending apparatus comprising a robotic armmoveable about multiple axes and a fixed gripping tool, comprising: apair of opposing gripping fingers moveable between open and closedpositions, said pair of opposed gripping fingers mounted to the end ofsaid robotic arm, said gripping fingers for gripping an orthodonticarchwire and thereby bending and/or twisting said orthodontic archwireby virtue of movement of said robotic arm while gripping saidorthodontic archwire arid while a first portion of said workpieceremains held by said fixed gripping tool; and wherein said pair ofopposed gripping fingers and fixed gripping tool are operative toadvance said orthodontic archwire through said fixed gripping tool in aseries of steps and bend successive segments of said orthodonticarchwire to thereby form a completed orthodontic archwire having apredetermined shape.